In order for a skeletal muscle to contract, your brain sends a signal, in the form of an action potential in an upper motor neuron.
The upper motor neuron is part of the cerebral cortex, and it activates a lower motor neuron, which is located in the anterior horn of the spinal cord.
From here, the action potential is sent through an axon down to its ending branches, called axon terminals, to muscle fibers which they innervate.
The place where an axon terminal meets the muscle fiber is the neuromuscular junction.
The neuromuscular junction has three main parts: a presynaptic membrane, which is the membrane of an axon terminal; a postsynaptic membrane, which is the membrane of a skeletal muscle fiber and is also called a motor end-plate; and a synaptic cleft, which is the gap between the presynaptic and postsynaptic membrane.
When an action potential reaches the axon terminal, it stimulates voltage-gated calcium channels in the membrane to open and extracellular calcium ions flow into the lower motor neuron.
Inside the axon terminal are synaptic vesicles that contain neurotransmitters called acetylcholine.
The calcium that enters the axon terminal binds to the vesicles, which allows them to fuse with the cell membrane of the axon terminal, releasing the acetylcholine into the synaptic cleft.
The acetylcholine then diffuses over to the motor end plate on the muscle fiber - and because it’s a short distance, that diffusion happens really quickly.
Here, two acetylcholine molecules will bind to one ligand-gated ion channel, also called nicotinic receptor.
When that happens, these ligand-gated ion channels, which are selective for positively charged ions, open up.
When they open, lots of sodium ions rush into the skeletal muscle fiber, and a few potassium ions leak out of the cell.
But overall there’s an increase in positive charge on the inside of the muscle fiber and therefore on the inside of the membrane, relative to the outside of the membrane - and this is called depolarization.